ENVI Glossary - ENVI Tutorial
ENVI Glossary 1 (A - N)

This glossary supplements ENVI Help by listing some terms that are frequently referenced in ENVI documentation or that are unique to ENVI functionality. It is not an exhaustive list of remote sensing terms. For terms that describe an ENVI function, see ENVI Help for complete details.


Advanced Along-Track Scanning Radiometer; an ENVISAT sensor.

absorption feature

A region of the electromagnetic spectrum where a given material absorbs radiation, shown by a low point in a spectral curve. Each material has unique absorption features, which serve as identifying characteristics.

abundance image

In hyperspectral analysis, an image whose values represent the fractions of total spectrally integrated radiance or reflectance of a pixel contributed by each spectrally unique material.


Australian Centre for Remote Sensing.

across track

The imaging direction perpendicular to the along track direction of a satellite or aircraft. The across track viewing angle determines the swath.

adaptive filter

A type of spatial filter that uses the standard deviation of those pixels within a local box (kernel) surrounding each pixel to calculate a new pixel value. Typically, the original pixel value is replaced with a new value. Unlike a typical low-pass smoothing filter, adaptive filters preserve image sharpness and detail while suppressing noise.

adjacency effect

The change in a pixel value caused by photons that reflect off the ground and scatter into the sensor field-of-view; radiance from neighboring pixels affects the measured radiance of a target pixel. The algorithms in the Atmospheric Correction Module in ENVI account for this effect.


ARC Digitized Raster Graphics; a National Geospatial-Intelligence Agency (NGA) data format.

aerosol scattering

Redirection of electromagnetic energy caused by aerosols, which are particles suspended in the atmosphere.


Airborne Synthetic Aperture Radar; a NASA Jet Propulsion Laboratory (JPL) sensor.

along track

The imaging direction parallel to the direction that a satellite or aircraft moves.

alpha residuals

In hyperspectral analysis, alpha residuals are spectra that are a function of emissivity only. They have a similar shape as emissivity spectra but have a zero mean.


A graphic element added to an image or map composition. Annotation objects include, but are not limited to, symbols, shapes, polylines, text, arrows, scale bars, borders, map keys, declination diagrams, color ramps, images, and plots.

apparent reflectance

Reflectance recorded at the sensor; also defined as radiance normalized by solar irradiance. Apparent reflectance is not true reflectance because shadows and directional effects have not been accounted for.

arbitrary profile

Also called a transect; a line drawn on an image, whose underlying data values are graphed in a cross-section plot.

area-based matching

A method of automatic image-to-image registration that compares the gray scale values of patches of two or more images and tries to find conjugate image locations based on the similarity of the gray scale value patterns.


Advanced Synthetic Aperture Radar; an ENVISAT sensor.


American Standard Code for Information Interchange; a simple text format.


The direction (azimuth) that a surface faces, typically in degrees clockwise from North (0 degrees); a parameter used in topographic modeling.


Advanced Spaceborne Thermal Emission and Reflection Radiometer; a NASA EOS sensor.

atmospheric correction

A method used to correct sensor radiance for atmospheric effects by mathematically modeling the physical behavior of radiation as it passes through the atmosphere. Following are some atmospheric correction methods available in ENVI:

  • Dark Subtraction: A method that uses the darkest pixel in a remote sensing image to remove path radiance and scattering effects. The method assumes that each band in an image contains some pixels at or close to a zero brightness value, and that atmospheric effects and path radiance add a constant value to each pixel in a band. Subtracting this constant value from the particular spectra removes the first-order scattering component. Dark subtraction, however, does not account for water vapor and ozone absorption.
  • Empirical Line: A method that forces image spectra to match reference spectra (field or laboratory) through linear regression.
  • FLAASH: Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes; an atmospheric correction modeling tool in ENVI for retrieving spectral reflectance from hyperspectral radiance images. FLAASH incorporates the MODTRAN4 radiation transfer model to compensate for atmospheric effects.
  • Flat Field: A method that calculates the mean spectra for a region assumed to have no spectral variation. The method divides each pixel spectrum by the Flat Field spectrum.
  • IAR: Internal Average Reflectance; a method that calculates the mean spectrum for the entire scene. The method divides each pixel spectrum by the scene-average spectrum.
  • QUAC: Quick Atmospheric Correction; an automated atmospheric correction method in ENVI for retrieving spectral reflectance from multispectral and hyperspectral images.
atmospheric window

A region of the electromagnetic spectrum in which satellites and aircraft cannot acquire measurements because of substances that absorb solar radiation (the most common are water vapor and CO2).


Along Track Scanning Radiometer; an ERS-1 and ERS-2 sensor.


Data that describe the properties of a point, line, or polygon record in a GIS. Attributes are typically stored in table format.


In multivariate statistics, a measure of correlation among residuals from a regression equation. An autocorrelation (r) value of 1.0 or -1.0 indicates a strong relationship between successive residuals, and a value of 0 indicates no relationship. In remote sensing, autocorrelation provides an indication of the local homogeneity of a data set, by evaluating the overall pattern between proximity and similarity of pixel values.


Advanced Very High Resolution Radiometer (NOAA).


Airborne Visible/Infrared Imaging Spectrometer (NASA/JPL).


A discrete portion of the electromagnetic spectrum measured by an aircraft or satellite sensor. Legacy multispectral sensors typically have fewer, wider bands, while modern hyperspectral sensors have hundreds of bands that each measure a very narrow range of wavelengths.


An algorithm developed by the Galileo Group, Inc., used to increase classification accuracy of targets in hyperspectral analysis. BandMax determines an optimal set of bands to help separate targets from known background materials.

Band Math

An ENVI tool that allows you to define and apply mathematical expressions to spatial image data, resulting in a new output image.

band ratio

The process of dividing one spectral band by another to enhance their spectral differences and to reduce the effects of topography.

batch mode

Performing a linear sequence of ENVI processing tasks in a non-interactive manner.

batch mode routine

An IDL program that includes ENVI library routines to perform a non-interactive image processing task.


Band-interleaved-by-line; an interleave format that stores the first line of the first band, followed by the first line of the second band, followed by the first line of the third band, interleaved up to the number of bands.


Band-interleaved-by-pixel; an interleave format that stores the first pixel for all bands in sequential order, followed by the second pixel for all bands, followed by the third pixel for all bands, and so forth, interleaved up to the number of pixels.


Band-sequential; an interleave format where each line of the data is followed immediately by the next line in the same spectral band. This format is optimal for spatial (x,y) access of any part of a single spectral band.


A user-specified extension zone around a point, line, or polygon. In a buffer zone image, each pixel represents the distance from that pixel to the nearest pixel of the selected class(es). Pixels that fall beyond a user-specified maximum distance threshold are set to that maximum distance.

byte order

The order of bytes in integer, long integer, 64-bit integer, unsigned 64-bit integer, floating-point, double-precision, and complex data types. Following are the two methods of byte order:

  • Host (Intel): Least significant byte first (LSF); byte order=0 in the ENVI Header; used in Intel-based (Windows, Linux, MacIntel) platforms.
  • Network (IEEE): Most significant byte first (MSF); byte order=1 in the ENVI Header; used in Macintosh and Unix platforms.

Compressed ARC Digitized Raster Graphics; an NGA data format.

change detection

The process of comparing two or more images acquired at different times.


Canadian Centre for Remote Sensing.


Controlled Image Base; an NGA data format.


The process of assigning pixels of a multispectral image to discrete categories. Following are some popular classification methods available in ENVI:

  • Supervised: A general technique that uses information derived from a few areas of known identity to classify unknown pixels in the remaining image.
    • Binary Encoding: A technique that encodes data and endmember spectra into zeros and ones, based on whether a band falls below or above the spectrum mean, respectively.
    • Maximum Likelihood: Pixels are assigned to the class in which they have the highest probability of being a member.
    • Minimum Distance: A technique that uses the mean vector of each endmember and calculates the Euclidean distance from each unknown pixel to the mean vector for each class.
    • Spectral Angle Mapper: An algorithm that determines the spectral similarity between two spectra by calculating the angle between them and treating them as vectors in a space with dimensionality equal to the number of bands.
  • Unsupervised: An automated technique that searches for natural groups, or clusters, of pixels based on their brightness in several bands. Unlike the supervised method, unsupervised classification does not begin with a predefined set of classes.
    • K-Means: A technique that calculates initial class means evenly distributed in the data space, then iteratively clusters the pixels into the nearest class using a minimum distance technique.
    • Isodata: A technique that calculates class means evenly distributed in the data space, then iteratively clusters the remaining pixels using minimum distance techniques.

The statistical analysis of a set of pixels to detect their inherent tendency to form clusters in n-dimensional (n-D) measurement space.

color-infrared composite

Also called CIR, or false-color composite; An image where the near-infrared band (0.76 - 0.9 μm) is displayed in red, the red band (0.6 - 0.7 μm) is displayed in green, and the green band (0.5 - 0.6 μm) is displayed in blue.

color ramp

An annotation object that shows a gradual transition from one color to another. For a gray scale image, the transition is from the minimum to the maximum gray scale value. For a color image, the color ramp is the distribution of the selected color palette.

color table

A special lookup table that associates screen brightness values with specific RGB values. For a color image, the output colors consist of different red, green, and blue values. For a gray scale image, the red, green, and blue data values are the same for a given data value.

color transform

A method used to convert RGB images to a different color space, and vice-versa; by applying a contrast stretch in another color space, you can highlight certain features in an image.

compound widget

In ENVI programming, a predefined widget that ENVI automatically builds for you when you call the appropriate library routine in your user function (see the ENVI Reference Guide for a list of library routines). Each compound widget performs a specific task that is often needed in a custom graphical user interface for image processing.

confusion matrix

Also called a contingency matrix. A table used to assess classification accuracy and misclassification between categories. The matrix is size m x m, where m is the number of classes. The rows in the matrix represent classes that are assumed to be true, while the columns represent classes derived from remote sensing imagery. The matrix also lists errors of commission and omission.

contour line

A line that follows the same elevation on a topographic map.

contrast stretch

A method of improving the contrast of a remote sensing image by stretching the original range of digital numbers (DNs) across the full contrast range of the display. Following are the most commonly used stretches in ENVI:

  • Equalization: Also called a histogram equalization stretch; you select the number of output gray scale classes (bins) to redistribute the data into, based on an image's histogram. The program assigns near-equal numbers of pixels into each bin. This type of stretch greatly enhances the most populated range of brightness values in the image and automatically reduces the contrast in the very light or very dark parts of an image.
  • Gaussian: A type of stretch based on a Gaussian curve, centered on a mean DN value that you specify. The range of data values that fall within a given standard deviation (that you specify) of the mean are stretched from 0 to 255.
  • Linear: A type of stretch that sets a minimum and maximum input value to 0 and 255, respectively. All values in between are linearly aligned to intermediate output values.
  • Square root: A linear stretch applied to the square root of a histogram.

The process of applying a spatial filter on an image, where each pixel is based on a weighted average of coefficients within an n x n matrix surrounding the pixel (where n is an odd number).


A statistical measure of the linear relationship between two data sets. If they vary in the same direction, the correlation is positive; if they vary in the opposite direction, the correlation is negative.


A plot of autocorrelation values at multiple lag distances; a measure of how autocorrelation decreases as distance increases. For example, if you specify a maximum lag distance of 5 pixels, autocorrelation is calculated for lags of 5, 4, 3, 2, and for each pixel's nearest neighbors.


A statistical measure of the tendency of two variables to move or vary together; more specifically, the simultaneous deviations of two variables from their means.

data dimensionality

The number of variables (bands) present in a data set.

data fusion

The process of displaying two data sets of the same area together in one RGB color composite. The data sets must be registered and resampled so that they have the same orientation, pixel size, and image dimensions. A popular example of data fusion is to apply an HSV color transform to one image, replace the value band with another image, then reverse the color transform. This produces an image that merges the color characteristics of one image with the spatial characteristics of another image.


A reference point or surface against which position measurements are made, and an associated model of the shape of the earth for computing positions. Different nations and agencies use different datums based on local reference points. Examples include the NAD83 and NAD27.

decision tree

A classification technique that uses a series of binary decisions to place pixels into classes. Each decision point divides pixels into two classes based on an expression. Then you can divide each new class into two more classes based on another expression, and so on.

declination diagram

An annotation object that includes any combination of arrows pointing to true north, grid north, and magnetic north.

delaunay triangulation

An image-to-image warping method that fits triangles to irregularly spaced tie points and interpolates values to the output grid.


Digital elevation model; a raster data set where each pixel represents an elevation value.

density slice

To convert the continuous gray tone of an image into a series of density intervals, or slices, each corresponding to a specific digital range.


A preprocessing method used to correct systematic distortions caused by earth rotation and scan skew; these were especially evident in Landsat MSS imagery.


A preprocessing method used to remove periodic scan line striping in image data. This type of striping is often seen in Landsat MSS data (every 6th line) and, less commonly, in Landsat TM data (every 16th line).


Digital Image Map; a SPOT data format.

display group

A term that refers collectively to the Scroll window, Image window, and Zoom window in ENVI.

display group menu bar

The menu bar in one window of a display group.


Digital Line Graph; a USGS vector data format.


Defense Meteorological Satellite Program (U.S. Air Force).


Digital number; also called pixel value.


Digital Orthophoto Quadrangle; a USGS data format.


Digital Raster Graphics; a scanned topographic map generated by the USGS.


Digital Terrain Elevation Data; an NGA data format.


Data Exchange Format; a format for storing vector data in ASCII or binary files.

dynamic overlay

An ENVI feature that allows you to immediately overlay and toggle (flicker) between two linked images.


Enhanced Compressed Wavelet; a proprietary data format developed by Earth Resource Mapping that is primarily intended for aerial imagery.


Transformation coefficients in principle components analysis that can be used to determine the percent of total variance explained by each of the principle components.


A set of weights applied to band values to obtain principal components; they show the relative contributions of the different original bands to the final principal components bands.

electromagnetic spectrum

The full spectrum of electromagnetic radiation, extending from short cosmic waves to long radio waves.


A smooth mathematical surface that is used to represent the geometric model of the earth. Its axes approximate the dimensions of the earth, rotated around the polar axis. Examples include the Clark ellipsoid of 1866 and the GRS80 ellipsoid.


The ratio of the radiance emitted from an object at a particular wavelength to the radiance that a blackbody would emit at the same temperature and wavelength. An emissivity of 1.0 is a perfect blackbody (a perfect "absorber"). Most earth surface types have emissivities between 0.9 and 1.0.


A pure spectrally unique material that occurs in a scene.


Empirical Flat Field Optimal Reflectance Transformation; an algorithm that "polishes" out noise and errors that may appear in hyperspectral apparent reflectance data, thus improving the accuracy of the data and making apparent reflectance spectra appear more like spectra of real materials.


Environment for Visualizing Images.

ENVI header file

A text file that must accompany an image file and reside in the same directory as the image file. The header file lists required image characteristics such as number of samples, number of lines, number of bands, offset, file type, byte order, data type, and storage order.

ENVI main menu bar

The main menu that appears when you start ENVI.

ENVI save files

Binary files that contain the basic ENVI library routines and internal variables required to run ENVI.

event handler

IDL code that manages events generated by widgets.


ENVI vector file; a format that ENVI uses to store vector data, no matter what the input native format is. EVF is the most efficient format for storing and manipulating vector information within ENVI.


Environmental Satellite (European Space Agency)


Earth Observing System (NASA)


Earth Observation Satellite Company

epipolar images

A stereo pair of images in which the left and right image are oriented such that ground control points (GCPs) have the same y-coordinates on both images, thus removing one dimension of variability. Epipolar images are generated based on epipolar geometry and are used to extract a DEM.


Earth Resources Observation System.


European Remote Sensing satellite (European Space Agency).


European Space Agency.


Environmental Systems Research Institute.


Enhanced Thematic Mapper; a Landsat-7 sensor.

exterior orientation

In photogrammetry, the process of transforming image coordinates to object (ground) coordinates using ground control points (GCPs).


The process of blending the edges of overlapping areas in input images for pixel-based and map-based mosaicking.


Also called spatial feature; A user-defined geographic phenomenon that can be modeled or represented using geographic data sets. Examples include roads, buildings, grasslands, and water bodies.


Fast Fourier Transform; a filter used to transform image data into a complex output image showing its various spatial frequency components.

fiducial marks

A series of four or eight crosshairs placed along the edge of aerial camera film during exposure. The intersection of imaginary lines connecting opposite fiducial marks corresponds to the principal point of the photograph. Fiducial marks are used primarily to orthorectify aerial photographs.

field spectra

Spectra of natural features such as minerals or vegetation, analyzed in the field using a handheld spectrometer. Field spectra are often used as a baseline, or "true" spectra for identification of minerals or vegetation types from hyperspectral remote sensing imagery.

flat binary

A general raster format where data are stored as a binary stream of bytes in BSQ, BIP, or BIL format.


Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes; a first-principles atmospheric correction modeling tool for retrieving spectral reflectance from hyperspectral radiance images. FLAASH is a separate add-on module in ENVI.


A 3D animation along a hypothetical flight path in a 3D SurfaceView.

frame central projection

A modeling scenario used to compute rational polynomial coefficients (RPCs) in frame camera and digital (frame central) aerial photography. This projection has one perspective center, which is collinear in space with its object point and image point.


In ENVI programming, a program unit containing one or more IDL statements that returns a value. Functions take the following form:

Result = FUNCTION_NAME(Argument [, Optional Argument])   

Full width half maximum; an engineering term that pertains to the signal curve of a sensor; in a plot of the measured signal (which approaches a near-Gaussian distribution), the FWHM is the horizontal distance between the two points on the signal curve that are half the maximum value. FWHM is an important measure of the quality of an imaging device and its spectral resolution.


In ENVI, a value that is multiplied by the pixel value to scale it into physically meaningful units of radiance:

radiance = DN * gain + offset   

Ground control point; a point on the ground whose location is known through a horizontal coordinate system or vertical datum. A GCP relates a point in a remote sensing image (x,y) to a geographic point on the earth (latitude/longitude, for example).


General Cartographic Transformation Package.

geographic coordinate system

A coordinate system for defining locations on the earth's surface using a 3D spherical model that includes an angular unit of measure, a prime meridian, and a datum.


An undulating surface that approximates the shape of the earth and mean sea level throughout the world. The direction of gravity is perpendicular to the geoid at every point. The geoid is the reference surface for surveying and some inertial navigation systems. An example is the OSU91A geoid.

geometrically corrected

An image that has been adjusted to remove geometric distortions caused by lens distortion, sampling rate variation, sensor drift, topographic relief, and other factors.


To map a remote sensing image to a known location on the earth, by referencing it to a map projection.


A public-domain metadata standard that allows geographic information to be embedded within a TIFF file. Remote sensing software uses the metadata to position the geographic data.


Geographic information systems.


Geographic lookup table; a binary file that maps an input pixel to an output pixel based on input geometry information. A GLT file contains integer pixel location values that are sign-coded to indicate if a certain output pixel is "real" or interpolated from nearest-neighbor resampling. The two bands of a GLT file refer to the original sample number and original line number, respectively. You can georeference your data directly from a GLT file.


Global positioning system.

gray scale

A range of black to white tones as displayed on a monitor or in an image; a gray scale image is created when the red, blue, and green color guns of the monitor are assigned the same value for each pixel.


Hierarchical Data Format; a data structure developed by the National Center for Supercomputing Applications.


Hierarchical Data Format - Earth Observing System; a format used for storing data from NASA EOS sensors that adds geolocation objects (grid, point, and swath) to the HDF format.

hill shade image

A color shaded-relief image created by transforming a color image into HSV color space, replacing the value band with a shaded-relief image, and transforming the color image back to RGB space.


A plot that shows the frequency of occurrence (along the vertical axis) of individual measurements or data values (along the horizontal axis); a frequency distribution.


Hue-lightness-saturation color space.


Hue-saturation-value color space.


A term used to describe data sets typically composed of 100 to 200 (or more) spectral bands of relatively narrow, contiguous, bands (5 to 10 nm). Hyperspectral imaging creates a large number of images from contiguous regions of the electromagnetic spectrum. This increases sampling of the spectrum (versus multispectral data) and greatly increases the amount of information available to a researcher.


Interactive Data Language.


Input Geometry file; an ancillary file that provides map information in two bands: one for x coordinates and another for y coordinates. Many data sets include IGM files in their distribution. An IGM file itself is not georeferenced, but it contains georeferencing information for each original, raw pixel in an image.


A GeoEye high-resolution satellite that produces 1 m panchromatic and 4 m multispectral imagery.

image box

The red box inside a Scroll window that defines the area covered in the Image window.

image coordinates

Also called pixel coordinates; the location of an image pixel in generic (sample, line) coordinates. Image coordinates always increase (one unit for every pixel) with increasing sample and line number.

image cube

A color image created from a multispectral or hyperspectral file where the data set is shown with an image represented on the face of a cube and the spectral information of the edge pixels depicted on the other faces.

image sharpening

The process of merging a low-resolution color image with a high-resolution gray scale image (with resampling to the high-resolution pixel size).

Image window

The window in a display group that displays the image at full resolution. If the image is large, the Image window displays the subsection of the image defined by the Scroll window Image box.

imaging spectrometer

A sensor designed to collect hyperspectral imagery. Examples include AVIRIS and HyMap. Many spectral images are acquired simultaneously, where each pixel in an image contains a continuous spectrum with typically hundreds of spectral measurements that is used to analyze surface features and atmospheric constituents.

incidence angle

In a radar system, the angle defined by the incident radar beam and the vertical (normal) to the intercepting surface. In pushbroom sensors, the along track incidence angle is the angle between the vertical position of the satellite and its forward or backward viewing direction. The across track incidence angle is the angle (in degrees) between the vertical position of the satellite and its side-viewing direction when the sensor is scanning along the side.

interactive user routine

In ENVI programming, a user function that performs some type of interactive analysis and is triggered by certain events or user selection. Examples include plot functions, Spectral Analyst functions, user-defined map projection types, user-defined units, user-defined RPC readers, and user move routines.

interior orientation

In photogrammetry, the process of transforming scanned image pixel coordinates to image coordinates defined by fiducial marks in the aerial photograph.


A term that refers to how raster image data are stored.


Indian Remote Sensing satellite (Government of India, Department of Space).


Japanese Earth Resources Satellite (Japan Aerospace Exploration Agency).


Joint Interoperability Test Command; an organization that certifies systems implementing the NITF data format for compliance with NITF standards.


Joint Photographic Experts Group image format.


Jet Propulsion Laboratory (NASA).


An array of pixels used for digital image filtering or to constrain an operation to a subset of pixels.

laboratory spectra

Spectra of materials (such as minerals or vegetation types) measured with a spectrometer in a controlled laboratory environment. Laboratory spectra are often used as a baseline, or "true" spectra for identification of materials from hyperspectral remote sensing imagery.


A surface that emits or reflects radiation isotropically, according to Lambert's law; a perfectly diffusing surface; the brightness (luminance, radiance) of a lambertian surface is constant regardless of the angle from which it is viewed.


A binary data format that contains LIDAR point data records.

layer stacking

The process of building a multi-band file from georeferenced images of various pixel sizes, extents, and projections. The output file has a geographic extent that either encompasses all of the input file extents or encompasses only the data extent where all of the files overlap.

library routines

IDL programs that encompass nearly all of the functionality in ENVI. The ENVI Reference Guide contains a complete index and full reference page for each library routine.


Light detection and ranging; a technology that determines distance to an object or surface by measuring the time delay between a laser pulse transmission and detection of the return signal.

line central projection

A model used to compute rational polynomial coefficients (RPCs) in imagery from pushbroom sensors and line central aerial photography. Each scan line has its own projection center.

line of sight

A spatial analysis tool that determines which pixels can be seen from a specific pixel within any file that has an associated DEM; topographic features will obscure some pixels from view.


The y component of a raster image coordinate pair (x,y); same as row.

logarithmic residual

An input spectrum divided by the spectral geometric mean (the mean of all bands for each pixel) of a data set, which is then divided by the spatial geometric mean (the mean of all pixels for each band). Logarithmic residuals are used to remove solar irradiance, atmospheric transmittance, instrument gain, topographic effects, and albedo effects from radiance data.


Lookup table; a static table that associates an image pixel value with a specific screen brightness value; used for contrast-stretching an image.

majority analysis

A post-classification tool used to change spurious pixels within a large single class to that class. You specify a kernel size, and the center pixel in the kernel is replaced with the class value represented by the majority of the pixels in the kernel.

map projection

A mathematical method of representing the earth on a flat plane. Hundreds of map projections are available to satisfy various project requirements (accurate distance, accurate navigation, equal area, etc.)


MODIS Airborne Simulator (NASA).


An image consisting of zeros and ones that, when applied to another image, tells ENVI which pixels in that image to analyze (ones) and which pixels to ignore (zeros). A mask is useful, for example, if you want to calculate image statistics while ignoring missing data values.


Medium Resolution Imaging Spectroradiometer; an ENVISAT sensor.


In the Spatial Feature Extraction module, merging refers to the process of joining adjacent segments based on the similarities between the features in these segments. Merging is an optional step used for solving certain over-segmentation problems.

meta file

A virtual file in ENVI that is essentially a combination of image files or bands treated as the same input file. An ENVI meta file is a text file with names and locations of files on disk. When you select the meta file for input or processing, ENVI retrieves the image data from the individual disk files and treats them as if they were part of the same input file for processing.

minority analysis

A post-classification tool where you enter a kernel size, and the center pixel in the kernel is replaced with the class value represented by the minority of the pixels in the kernel.

MNF rotation

Minimum Noise Fraction; a transform used to determine the inherent dimensionality of image data, to segregate noise from the data, and to reduce the computational requirements for subsequent processing. MNF rotation consists of two principal component transformations, with a noise whitening step.


Moderate Resolution Imaging Spectroradiometer; a NASA EOS sensor aboard the Aqua and Terra satellites.


An atmospheric radiative transfer model developed by the Air Force Research Laboratory that predicts path radiances and transmissions through the atmosphere and can be used to describe vertical profiles of water vapor, ozone, and aerosols. The FLAASH module uses MODTRAN code.

morphological filter

A filter based on mathematical morphology that changes the shape and connectivity of an object. Unlike a convolution filter that multiplies neighborhood pixels by values you specify within a kernel, a morphological filter only works with the data in the neighborhood itself and uses either a statistical method or mathematical formula to modify the pixel upon which it is focused. The most common morphological filters are dilation, erosion, opening, and closing.


A set of overlapping aerial or satellite-based images whose edges are matched to form a continuous pictorial representation of a portion of the Earth's surface.


Moving Picture Experts Group; a data format for digital audio and video.


Multi-Resolution Land Characteristic; a Landsat TM and DEM data format.


Multispectral Sensor; a Landsat sensor.

multiband file

A digital image that contains more than one band of data.


A method for reducing speckle noise in synthetic aperture radar (SAR) data and for changing the size of a SAR file by averaging neighboring pixels throughout the image.


The ability of a remote sensing instrument to detect wavelengths in two or more spectral bands.


The point on the ground that lies vertically beneath the perspective center of the aerial camera lens or satellite sensor.


Normalized Difference Vegetation Index; a vegetation index used to transform multispectral data into a single image band whose values indicate the amount of green vegetation present in the pixel. ENVI uses the standard NDVI algorithm, where NIR is a near-infrared band:

NDVI = (NIR - Red / NIR + Red)   

National Imagery Transmission Format.


National Landsat Archive Production System; a Landsat TM and MSS data format.


National Marine Electronics Association data format; the NMEA-0183 format is commonly used with a GPS.


National Oceanic and Atmospheric Administration.


To transform a set of measurements so they may be compared in a meaningful way. Normalization commonly refers to rescaling minimum and maximum values between two or more data sets so all of the values range from 0 to 1, allowing the data sets to be directly compared.


NATO Secondary Image Format; a data format similar to NITF that is used by members of the North Atlantic Treaty Organization (NATO).